Side chain engineering of [1]benzothieno[3,2-b]benzothiophene (BTBT)-based semiconductors for organic field-effect transistors

[1]benzothieno[3,2-b]benzo-thiophene (BTBT)-based compounds with different side chains, 2-(2-ethylhexyl) benzo[b]benzo[4,5]thieno[2,3-d]thiophene (compound 1), 2-(2-ethylhexyl)-7-(thiophen-2-yl)benzo[b]benzo [4,5]thieno[2,3-d]thiophene (compound 2), 2-(2-ethylhexyl)-7-(5-octylthiophen-2-yl)benzo[b]benzo[4,5] thieno[2,3-d]thiophene (compound 3), and 2-(2-ethylhexyl)-7-(5-(2-ethylhexyl)thiophen-2-yl)benzo[b]benzo [4,5]thieno[2,3-d]thiophene (compound 4), were synthesized and characterized as solution-processable small molecular organic semiconductors for organic field-effect transistors (OFETs). The corresponding compounds were employed as active layers for top-contact/bottom-gate OFETs via solution-shearing method and the electrical properties were investigated. The resulting devices exhibited p-channel activity and, especially, thin films of monoalkylated BTBT with additional thiophene ring showed the highest hole mobility up to 0.12 cm2/Vs and current on/off ratio over 107. The analysis of atomic force microscopy (AFM) and X-ray diffraction (XRD) were carried out to correlate surface morphology and microstructure of thin films with the device performance.